Modular artificial hip joint

ABSTRACT

A modular artificial hip joint includes a head part and at least one shaft part. The shaft part, which can be driven into the bone and forms the distal region, is connected to the distal end of the head part by insertion, preferably by means of an insert cone. A first screw or tie rod, guided through an axial bore in the head part, can be screwed into the threaded bore in the shaft part. In order to guide a second screw for separating the conical insert connection between the individual components of the artificial joint, the bore in the head part is threaded and has a diameter which is greater than the diameter of the threaded bore in the shank part.

BACKGROUND OF THE INVENTION

The invention relates to a modular joint prosthesis having a head partand at least one shaft part which are insertably connectable with oneanother by means of a cone connection.

Such joint prostheses are produced in the most various shapes and sizes,to enable the most precise possible adaptation to the shape of the boneof a particular patient, as well as the current status of the bonytissue, taking the clinical picture into account.

By means of a multi-part embodiment with nonpositive connection of thecorresponding individual parts in the proximal region, the adaptationcan be accomplished optimally. At the same time, positioning of thejoint head is possible regardless of the shaft diameter.

From European Patent Disclosure EP-B1 0 243 298, a kit for a shaftprosthesis is known, which has a head part that can be provided with ajoint ball, an end part anchored in the bone, and an intermediate partthat can be positioned between the two. All the parts have conical boresor complementary pegs, and as a result the prosthesis can be assembledby making conical insert connections. The head part and intermediatepart each have an axial through bore.

The end part is provided with releasable engagement means, for the sakeof transmitting a force in the axial direction. When the individualparts are put together, the corresponding bores are aligned axially inthe direction of the shaft. The individual parts of the prosthesis areput together using a tie rod that transmits a force in the axialdirection and that penetrates both the head part and succeeding shaftparts and can be screwed into the threaded bore of the end part. As aresult, the head part, or the intermediate part, and the end part arefirmly tightened against one another, so that loosening of theindividual parts of the prosthesis from the mechanical strain during useneed not be feared.

The above-described embodiment has the disadvantage that in such aprosthesis, the insert connection between the individual parts cannot beundone or released in the implanted state, even though this is sometimesnecessary for medical reasons—for instance for resetting or replacingthe head part. The conical insert connections, even after prior removalof the tie rod provided in the interior of the shaft, are held togetherso firmly that, if disassembly becomes necessary, damage to the femur orat least undesired loosening of the shaft is sometimes unavoidable,which is detrimental to the patient.

SUMMARY OF THE INVENTION

In view of the deficiencies in the prior art, it is the object of theinvention to create a modular joint prosthesis of the generic typedescribed at the outset, which in a simple way allows the individualelements to be loosened from one another, even from the outside.

This object is attained according to the invention by providing a borethat extends essentially coaxial to the longitudinal axis of a shaftpart of a modular joint prosthesis and through the head part of themodular joint prosthesis and at least one section of the shaft part, thebore having a first segment with a first diameter and a first thread inthe head part and a second segment in the shaft part with a secondthread part and a second diameter, where the first diameter is greaterthan the second diameter so that an insert connection between the headpart and the shaft part can be released.

The invention encompasses the recognition that with a screw bolt,considerable pressure forces can be generated in the axial direction ifthe end of the bolt is braced against an abutment. To that end, the edgeof the bore of a succeeding shaft element is also suitable, on thecondition that the outer diameter of a bolt to be screwed into thethreaded bore is greater than the inside diameter of the succeedingelement. Since replacement, especially of the head element, may becomeadvisable (in advanced disease or for other indications), the succeedingelements may have a bore of constant diameter; in that case, the last(distal) element optionally has a thread that has a tie rod forconnecting the shaft elements or setting the cone elements. The exertionof force for loosening two successive elements can be achieved all themore conveniently, the less its pitch is selected to be.

It is especially advantageous in the invention that without removing thelower part of the shaft, which remains in the bone space, the head partnear the joint can be removed and replaced if needed. This is associatedwith substantially less stress on the patient than in conventionalprostheses. Moreover, with the head part removed the correspondinginterior of the bone is also accessible for other therapeutic purposes.“Head part” in the present afflication is understood to mean the part ofthe prosthesis near the joint, a part that can also be advantageouslyused in the case of joint replacement in other regions of the body.

Prosthesis shafts embodied as a modular system have, at least as thehead part and end part, a system of various individual elements ofdifferent sizes, which are preferably joined together by puttingtogether their proximal or distal ends that have corresponding conicalpegs or recesses. The requisite stability of the insert connection isassured by screw means—preferably embodied as a tie rod. The end part isembodied as a hollow shaft, and the axially extending longitudinal boreof the carrier part has a thread on its proximal end portion. The tierod is guided through a cylindrical channel, located on the same axis asthe longitudinal bore in the carrier part, in the head part and screwedinto the end part, which is provided with a thread.

According to the preferred embodiment of the invention, at least part ofthe cylindrical channel in the head part of the modular hip prosthesisshaft is embodied as a threaded bore. This threaded bore has a diameterwhich is greater than the diameter of the threaded bore, receiving thetie rod, in the succeeding shaft part. This diameter relationshipadvantageously assures that when the prosthesis parts have been puttogether, a free area remains on the proximally provided, frustoconicalend of the succeeding shaft part, this free area being an essentiallycircular-annular top face of the truncated cone. This face region isavailable as an abutment for a screw bolt, which can be screwed into thehead part once the tie rod is removed. The bolt screwed into the headpart is braced on the substantially circular-annular abutment. Upon afurther screwing motion, an axial pressure force is favorably generated,which releases the firm conical insert connection (despite the absenceof the tie rod) between the head part and the succeeding shaft partenough that the head part can be removed for replacement purposes, orcan be swiveled to the desired extent with regard to the succeedingshaft part on its frustoconical peg provided on the proximal end. Toassure an adequately large abutment area for force transmission when theconnection between the head part and the succeeding shaft part isloosened or released, a diameter ratio between the bores in the headpart and the succeeding shaft part in the range from 1.5 to 2.5 isfavorable.

In another advantageous further feature of the invention, the threadprovided for the screw bolt extends in the head part of the modular hipprosthesis shaft over the entire length of the available cylindricalchannel. For both the threaded bore in the head part of the hipprosthesis shaft and for the threaded portion on the proximal end of thesucceeding shaft part, a metric thread is provided. The thread in thehead part preferably has a lesser pitch than the thread on the proximalend of the succeeding shaft part and thereby makes it easier to generatethe pressure force, by means of the screw bolt, that is required toloosen the firm conical insert connection.

In a further advantageous feature of the invention, the describedprovisions can be employed, using various graduations, multiple times insuccessive elements with one and the same modular shaft. In thedescription made here, the term “head part” would merely then bereplaced by the phrase “preceding shaft element in the distaldirection”. It can be seen that the graduations in diameter of the boreseach increase in the distal direction.

In an additional further feature of the invention, the proximal end ofthe threaded bore provided in the succeeding shaft part has a recess,which is favorably embodied as a chamfer arranged symmetrically to thelongitudinal axis of the bore. It is thus advantageously achieved thatthe screw bolt, whose free end is likewise chamferred at the same angle,when braced upon the proximal end of the supporting part cannot enterinto operative contact with the thread of the threaded bore provided inthis region, and thus when the pressure force required to loosen theconical insert connection between the head part and the succeeding shaftpart is generated, deformation of the first courses of the thread isavoided. A deformed thread would make it impossible to screw the tie rodback into the proximal end of the succeeding shaft part after adaptationor replacement of the head part. For introducing the force into theabutment, an angle of inclination of the chamfer in the range from 45°with respect to the center axis of the corresponding threaded bore isfavorable.

The head part, on its medial side, below the cone for the jointconnection, has a channel recess, which favorably forms an engagementand support region for a tool that is inserted to swivel or pull off thehead part, once the cone connection between the head part and thesucceeding shaft part has been released as described above.

To assure the maximum possible adaptation of the joint shaft to theanatomical conditions of a patient, a kit with differently embodied headparts and succeeding shaft parts for producing an individually adaptedshaft is advantageous. The individual parts of the hip joint shaftdiffer in diameter, length and curvature. They can be combined with oneanother, on the condition of their having an adapted outer diameter.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantageous further features of the invention are defined by thedependent claims and will be described in further detail below, togetherwith the description of the preferred embodiment of the invention, inconjunction with the drawings in which:

FIG. 1 is a the preferred embodiment of the invention in a schematicfragmentary sectional view;

Fig. 1ais the sectional view taken along the line 1 a. . . 1 a of FIG.1;

FIG. 2 is a simplified view of the preferred embodiment of the inventionof FIG. 1, in a side view seen from the left;

FIG. 3 is an enlarged view of the detail E in FIG. 1.

FIG. 4 shows a tie rod being screwed into a bore of the preferredembodiment of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The modular joint prosthesis 1, shown in FIGS. 1, 1 a and 2 in the formof a side view, a section, and a fragmentary section, comprises a shaftpart 2 and a head part 3, each of which have profiling in thelongitudinal direction. The individual parts 2 and 3 of the shaft 1 arejoined together, located on the same axis and pivotable relative to oneanother, by means of a conical insert connection 10. For “setting” thecone and securing this insert connection against axial loosening, a tierod is provided, which is passed through the channel 12, embodied as athreaded bore, of the head part 3 and screwed into the proximal region,embodied as a threaded bore, of the channel 4 provided in the succeedingshaft part 2 FIG. 4. The screw insertion proceeds far enough that theproximal end of the tie rod is braced in the recess 7, and theindividual parts 2 and 3 of the modular hip prosthesis shaft 1 are movedaxially toward one another, until the conical insert connection 10 hasthe desired strength.

In order to be able to vary the position of the head part 3 which has ajoint connection cone 9, relative to the succeeding shaft part 2 or toreplace the head part 3, the conical insert connection 10 must beloosened again. After the tie rod, 12 is loosened and removed bytwisting, a screw bolt (not shown) is screwed into the threaded bore 5of the head part 3. Since the diameter D₁ of the threaded bore 5 in thehead part 3 has a greater value than the diameter D₂ of the proximalportion 6, provided with a thread, of the central bore 4 of thesucceeding shaft part 2, the screw bolt is based against a circularring, whose area is defined by the difference in diameter D₁-D₂, on thetop face 8 of the proximal end, embodied as a truncated cone, of thehead part 3 and is thus capable of generating an axially orientedpressure force. This force releases the insert connection 10, which isvery firm after the “setting” of the cone, and conveniently makes itpossible to readapt the hip prosthesis shaft 1 to a patient's alteredphysical conditions.

The channel recess 9 a provided on the medial side of the head part 3below the joint connection cone 9 serves as an engagement point for atool, with which the head part 3 after the conical insert connection 10between the head part and the succeeding shaft part has been loosenedagain, can be pivoted on the same axis or pulled off the succeedingshaft part if needed.

In this connection, it has proved favorable for the manipulability ofthe means for generating the pressure force required to loosen theconical insert connection to provide a diameter ratio of the bores 4, 5in the range from 1.5 to 2.5, and to provide the bore 5 with a threadover its entire length. Metric threads are preferably used; the threadof the threaded bore 5 in the head part 3 has a lesser pitch than thethread in the proximal portion 6 of the bore 4 of the succeeding shaftpart 2.

The detail E of FIG. 1, shown on a larger scale in FIG. 3, illustratesan advantageous further feature of the invention shown in FIGS. 1, 1 aand 2. To prevent an effective intervention of the screw bolt (notshown) on the top side of the proximal end of the succeeding shaft part2 from damaging the threaded portion 6 provided there, a recess 11embodied as a chamfer is provided on the proximal end of the threadedbore. The inclination of the flanks 12 amounts to 45° and corresponds tothe inclination of the flanks of the chamfer on the threaded end of thescrew bolt (not shown). As a result, an optimal introduction of forceinto the succeeding shaft part 2 can be accomplished, in order to enableloosening the conical insert connection 10 between the succeeding shaftpart 3 and head part 2 with a relatively slight exertion of force. Inthis way, the head part can be removed from an implanted shaft, if alater operation should become necessary, without a substantial exertionof force and with only slight reaction forces. This makes it quitecertain that the shaft part that remains in the bone will not loosen orcome out of the bone.

It will be appreciated that—in further embodiments not shown in thedrawing—by means of the attained modularity in a system of shaftprostheses for attaining different individually dimensioned prosthesisforms, various sizes of elements can be combined with one another. Thegraduation in thread diameters provided according to the invention canlikewise be provided between different successive elements. Thus it ispossible to achieve a division—if desired—not only between the head partand the distally succeeding shaft part but also between different shaftparts, for instance. For purposefully “dialing up” a connection that isto be released, with thread diameters that increase in stages toward thehead end, one would then have to look for a bolt whose male thread comesinto engagement with the female thread, which comes into engagement withthe shaft element located on the proximal side of the intended dividingpoint. It is then braced by the edge of its end on the edge of the boreof the distally adjacent element and separates the selected connection.

In a modular system with differently curved shaft elements, individuallyshaped shafts can be created by means of different relative anglepositioning, without in any way limiting the releasability of theconnection as described above.

The invention is not limited in its realization to the preferredexemplary embodiment described above. On the contrary, a number ofvariants are conceivable, which make use of the provisions described,even in fundamentally different types of embodiments.

What is claimed is:
 1. A modular joint prosthesis having a head part andat least one shaft part which can be connected by means of an insertablecone connection, wherein a bore is provided that extends essentiallycoaxial to a longitudinal axis of the shaft part and extends through thehead part and at least one section of the shaft part, said bore having afirst segment with a first diameter and a first inside thread in thehead part, and a second segment with a second inside thread in the shaftpart and a second diameter which is 1.5 to 2.5 times smaller than thefirst diameter, so as to define circular ring on the shaft part betweenthe first and second segments, whereby a screw means, screwed into thefirst inside threads, can generate an axially oriented pressure force onthe circular ring to sever the insertable cone connection; and a tie rodhaving a proximal end, said tie rod being inserted into the bore andscrewed into the second inside thread in order to set and secure theinsertable cone connection, until the proximal end of the tie rod issupported on the head part.
 2. The joint prosthesis of claim 1, whereinthe first inside thread and the second inside thread extend over theentire length of their respective segments of the bore.
 3. The jointprosthesis of claim 1, wherein the shaft part has a proximal endadjacent a region where the shaft part connects to the head part and thesecond inside thread is provided only in the region of the proximal end.4. The joint prosthesis of claim 1, wherein the bore of the secondsegment has a chamfer which is adjacent the first segment of the bore.5. The joint prosthesis of claim 4, wherein the chamfer has an angle ofinclination of approximately 45°, with regard to a center axis of thesecond segment.
 6. The joint prosthesis of claim 1, wherein the headpart further includes a joint connection cone on a medial side of thehead part; and a channel recess below the joint connection cone, saidchannel recess having an arc-shaped boundary.
 7. A kit for making amodular joint prosthesis, comprising: a selection of head parts havingat least one of a different length and a different diameter; and aselection of shaft parts having at least one of a different length, adifferent diameter and a different curvature where the head parts andthe shaft parts are connected by means of an insertable cone connection,and a bore is provided that extends essentially coaxial to alongitudinal axis of a shaft part and extends through a head part and atleast one section of the shaft part, said bore having a first segmentwith a first diameter and a first inside thread in the head part, and asecond segment with a second inside thread in the shaft part and asecond diameter which is 1.5 to 2.5 times smaller than the firstdiameter so as to define a circular ring on the shaft part between thefirst and second segments; and a tie rod that can be inserted into thebore and screwed into the second inside thread in order to set andsecure the insertable cone connection, until the proximal end of the tierod is supported on the head part.
 8. A kit of claim 7 wherein adiameter in the range from 12 to 17 mm is contemplated for the headparts, and a length range from 200 to 320 mm and a diameter range from10 to 14 mm are provided for the shaft parts.